1. Field of the Invention
The present invention relates to printing systems which provide several alternative inks for printing. More specifically, the present invention concerns a printing system in which low-penetration black ink and high-penetration black ink are selectably used to print black pixels upon a recording medium.
2. Description of the Related Art
Conventional ink jet printing systems have heretofore employed several different methods to print black ink upon a recording medium. Particularly, black color may be printed using either low-penetration black ink, high-penetration black ink, or so-called process black (hereafter PCBk). It should be understood that, throughout the present specification, the terms "high-penetration" and "low-penetration" are not intended to describe absolute amounts of ink penetration into a recording medium. Rather, the term "high-penetration ink", as used herein, merely indicates an ink such as dye-based ink having a relatively greater penetration into a recording medium, whereas those inks described herein as "low-penetration" are inks such as pigment-based or evaporation inks having a relatively lower penetration.
Low-penetration black ink is often used to print black color in situations where a user desires high contrast between a printed black region and a lighter-colored surrounding region. This situation most commonly arises during text printing. Accordingly, most monochrome ink jet printers utilize low-penetration black ink.
High-penetration black ink is conventionally used in conjunction with differently-colored inks in order to produce color images. In this regard, a color printer usually provides for high-penetration inks corresponding to the subtractive color components, cyan, magenta, and yellow, as well as for black ink, which is used for under color removal. Variously-colored pixels are produced by printing none or some or all of the four inks at a single location upon a recording medium. Mixing of the printed inks generally produces muddy colors so high-penetration inks, which do not mix thoroughly, are generally preferred for color printing. Since high-penetration color inks are used for color printing, high-penetration black ink, which produces images having an optical density near to that of the color inks, is preferred for printing black pixels within a differently-colored region.
PCBk is produced by combining equal amounts of full-intensity cyan, magenta and yellow inks. In practice, PCBk does not provide as satisfactory a black color as low-penetration black ink or high-penetration black ink. However, PCBk, which shares penetration characteristics similar to its component colored inks, provides an output density similar to that of differently-colored pixels and is therefore useful for printing black color where uniform density between a black region and a color region is desired.
Therefore, whether a black pixel within an image should be printed using low-penetration black ink, high-penetration black ink, or PCBk is influenced by the content of the image data proximate to the black pixel. In this regard, current printing systems analyze binarized image data adjacent to a black pixel in order to determine a method for printing the black pixel. Unfortunately, in a case where the binarized data is based on multi-level original image data, the binarized data may not accurately reflect the original image data. As a result, a determined method for printing the black pixel may be inappropriate.
Moreover, as can be understood from the foregoing, optical characteristics of printed ink depend substantially upon an amount of penetration of the ink into a recording medium. However, conventional ink jet printing systems do not provide for selectable printing of low-penetration black ink and high-penetration black ink based upon absorption or other characteristics of a recording medium.
Therefore, what is needed is a printing system which selectably provides high-penetration black ink and low-penetration black ink for printing black pixels upon a recording medium based upon multi-level image data and/or characteristics of a recording medium.
On a related note, due to bleeding that occurs at a boundary between high and low-penetration inks, PCBk is sometimes used in order to provide a transition between a black region printed using black ink and a differently-colored region. Such a technique is disclosed in commonly-assigned co-pending application Ser. No. 08/901,719, entitled "PRINT DRIVER FOR A COLOR PRINTER", filed Jul. 28, 1997.
However, in a case that PCBk is used at a boundary between a black region printed using low-penetration ink and a differently-colored region, the boundary is distracting to the eye. This distraction is caused by differences between the optical density of the differently-colored region, printed using high-penetration ink, and the optical density of the black region, printed using low-penetration ink.
On the other hand, in a case that PCBk is used at a boundary between a black region printed using high-penetration ink and a differently-colored region, the printed black region consists of a low-contrast, dull black color. Accordingly, each of the foregoing methods for printing a boundary region between black regions and differently-colored regions are unsatisfactory.